1. Field of the Invention
The present invention relates to a power tool with a driving motor and more particularly, to a technique for controlling the output characteristic of the motor by adjusting the number of turns of a wire of a coil forming an armature winding.
2. Description of the Related Art
Generally, an armature of a DC motor has a plurality of slots. Coils are formed by winding a wire a number of turns between the slots. The coils are connected to associated segments of a communicator and define armature windings. The driving current of the motor is supplied to the coil of the armature windings via the segments and brushes which are in sliding contact with the segments. Upon such supply of current, a magnetic field is generated around the coils and interacts with the field generated by a stator which is fixedly disposed around the armature.
In the known DC motor, the same number of brushes as the number of poles of the stator is provided. However, the resistance loss caused by friction between the commutator and the brushes during the rotation of the armature may increase with increase in the number of brushes. Further, the number of parts increases as the number of brushes increases. In this connection, Japanese non-examined laid-open Patent Publication No. 2-184246 discloses a motor which can be driven while having a four-pole stator and two brushes by short-circuiting diametrically opposed segments of a commutator such that the coils connected to the opposed segments are short-circuited,
All of the coils that form armature windings consist of the same number of turns of a wire wound between slots, which number is selected according to a desired output characteristic of a motor, i.e. a desired torque or rotation speed. If a different number of turns is selected for each coil, the magnetic field generated around each of the coils when the driving current passes through the coils will wary in strength, because each of the coils that form armature windings is connected to associated segments. As a result, the commutator is deteriorated and suffers degradation in performance